Single and multiple quantum well layer structures have become increasingly important in the areas of optical communication systems and optical computing systems. These structures exhibit linear and nonlinear properties which permit them to be used as logic elements in switching system applications as well as modulation, detection and gain elements in comunication system applications. See, for example, U.S. Pats. Nos. 4,525,687 and 4,546,244.
In general, these device structures operate on an absorption principle. That is, the device is illuminated by a light beam from an external source at an optical wavelength at or above the absorption edge of the quantum wells. By electrically manipulating the absorption edge of the quantum well material, it is possible to absorb or transmit the light incident on the device. When the light is absorbed in the quantum wells, a voltage may be generated by the photovoltaic effect. Absorbed photons generate mobile charge carriers that move under the action of an applied electric field. Movement of the charge carriers changes the electric field and, thereby, generates a change in voltage. This mode of operation is useful in generating a steady current provided that the light is selected to have a wavelength that is strongly and continuously absorbed by the quantum wells, i.e., the photon energy of the incident light is at or above the bandgap energy of the quantum wells. While this mode of device operation is useful for detecting relatively long optical signal pulses, it severely limits the applicability of the device to very high speed optical pulse detection, among other things, because of the persistence of the absorption induced photovoltage long after the extinction of a high speed (short) optical signal pulse.